Optical disk apparatus and its reproducing method

ABSTRACT

Amplitude and frequency of a high-frequency signal superposed on a reproduction laser beam are changed in accordance with a reproduction spot diameter on the surface of the recording layer upon discrimination of an optical disk in initial adjustment after insertion of the optical disk and change of a layer of a multi-layer optical disk. Further, the high-frequency signal is not superposed on the reproduction laser beam until end of the discrimination and change to a target layer upon discrimination of the optical disk and change of a layer of a multi-layer optical disk.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese application JP2006-266353 filed on Sep. 29, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to control of laser waveform uponreproduction of information in an optical disk apparatus.

In an optical disk apparatus using a laser beam emitted from a laserdiode to read information of mark and space recorded in an optical disk,it is important to decide binary information of mark and space withouterror. However, there is a problem that noise such as mode hop noise andreturn light noise is produced in the laser beam.

As a method of reducing such noise, JP-A-2000-149302 discloses a methodnamed high-frequency superposition. Further, as a method of controllingthe laser power upon reproduction, JP-A-2005-216395 discloses a methodnamed automatic power control (APC).

Recently, optical disks having different standards such as CD, DVD,Blu-Ray disc (hereinafter abbreviated to BD) and HD-DVD are taken to themarket. These optical disks are not compatible with one another,although optical disk apparatuses dealing with recording andreproduction for CD, DVD and BD and optical disk apparatuses dealingwith recording and reproduction for CD, DVD and HD-DVD are taken to themarket in consideration of user's convenience. However, an optical diskapparatus dealing with recording and reproduction of both of BD andHD-DVD is not taken to the market yet.

It is considered as a cause of the reason that there are differentpoints on the standards between the optical disks conforming to the BDstandards (hereinafter referred to as “optical disks 1”) and the opticaldisks conforming to the HD-DVD standards (hereinafter referred to as“optical disks 2”) as shown in FIG. 4. The relation of the wavelength ofthe laser beam (405 nm for both of BD and HD-DVD) and the numericalaperture of lens (0.85 for BD and 0.65 for HD-DVD) prescribed in thestandards is considered as being particularly a problem among them.

When the wavelength of the laser beam is λ and the numerical aperture oflens is NA, it is known that a spot diameter of the laser beam on theoptical disk is proportional to:

(λ/NA)  (expression 1)

and a spot area on the optical disk of the laser beam is proportionalto:

(λ/NA)²  (expression 2)

That is, the spot areas on the optical disk are different between the BDstandards and the HD-DVD standards. The reproduction laser power valuesfor the BD standards and the HD-DVD standards are made different to be0.3 mW and 0.5 mW, respectively, so that the power of the laser beam perunit area is made substantially equal.

When the characteristics (amplitude and frequency) of the high-frequencysignal superposed in order to reduce noise of the laser beam are fixed,satisfactory reproduction can be made for one optical disks, althoughthere arises a problem that satisfactory reproduction cannot be made forthe other optical disks and more particularly there arises a problemthat data recorded on the optical disk is destroyed.

SUMMARY OF THE INVENTION

The problem is solved by the present invention described in Claims.

According to the present invention, erasure of recorded data ordestruction of the information recording layer caused by irradiating theinformation recording surface of the optical disk with excessive laserpower upon information reproduction can be prevented for two kinds ofoptical disks having the same laser wavelength for use in reproductionand the different numerical apertures.

The causes for the above problem are now examined in detail.

When the optical disks having the standards that the wavelengths of thelaser beam are the same and the numerical apertures of lens aredifferent as the relation of the BD standards and the HD-DVD standardsare defined as an optical disk 1 and an optical disk 2, respectively,and the relation of the numerical apertures of lens NA₁ and NA₂ isNA₁>NA₂, the relations of the spot areas S₁ and S₂ of the laser beam onthe optical disks and the reproduction laser powers P₁ and P₂ are givenby the following two expressions:

S₁<S₂  (expression 3)

P₁<P₂  (expression 4)

There is considered an optical disk apparatus which can makereproduction for both the optical disks 1 and 2.

It will be understood from the expression 4 that the reproduction laserpower is required to be changed when the optical disk 1 is reproducedand when the optical disk 2 is reproduced.

At this time, when a high-frequency signal having a preferable amplitudefor the optical disk 2 is superposed in reproduction of the rewritableoptical disk 1, there is a possibility that the irradiation energy perunit area reaches the erasure power of the optical disk 1, so thatrecorded data is erased. In other words, although the amplitude setvalue of the high-frequency signal does not cause the problem for alarger spot area S₂ on the optical disk 2, the problem is caused for thesmaller spot area S₁ on the optical disk 1.

The cause is considered that energy per unit area given by thereproduction laser power P₁ and the peak power of the high-frequencysignal superposed thereon is equal to energy per unit area given by theerasing laser power for the optical disk 1, so that recorded data iserased. This is considered to be the cause of the above-mentionedproblem that the recorded data on the other optical disk is destroyed byirradiation of the excessive laser power.

On the other hand, when attention is paid to the specific laser, it hasbeen found that the optimum amplitude and frequency of thehigh-frequency signal superposed in order to reduce laser noise dependon the laser power. That is, it has been found that it is desired tochange the amplitude and frequency of the high-frequency signal when thelaser power value is changed.

From the foregoing, when the numerical apertures are different in twokinds of the standards for optical disks using the laser having the samewavelength, that is, when it is necessary to change the reproductionlaser power, it has been found that the characteristics (amplitude andfrequency) of the high-frequency superposition are also required to bechanged.

Embodiments for solving the above problems are now described in detail.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an optical disk apparatusaccording to a first embodiment;

FIG. 2 is a block diagram schematically illustrating an internalconfiguration of a laser driver in the first embodiment;

FIG. 3 is a graph showing the relation of numerical aperture NA of alaser beam and spot diameter R and spot area R formed on an opticaldisk;

FIG. 4 is a table showing parameters for two kinds of optical diskshaving the same reproduction laser wavelength and different numericalapertures;

FIG. 5 is a graph showing the relation of laser driving current andlaser beam power by the driving current;

FIG. 6 is a schematic diagram showing laser beam waveform for an opticaldisk 2 in the first embodiment;

FIG. 7 is a schematic diagram showing laser beam waveform for an opticaldisk 1 in the first embodiment;

FIG. 8 is a flow chart showing a processing procedure to startreproduction of the optical disk in the first embodiment;

FIG. 9 is a schematic diagram illustrating an optical disk apparatusaccording to a third embodiment;

FIG. 10 is a block diagram schematically illustrating an internalconfiguration of a laser power control circuit and a laser driver in thethird embodiment;

FIG. 11 is a flow chart showing a processing procedure to startreproduction of the optical disk in the third embodiment;

FIG. 12 is a flow chart showing a processing procedure to startreproduction of the optical disk in a fourth embodiment; and

FIG. 13 is a block diagram schematically illustrating an internalconfiguration of a laser driver in a second embodiment.

DESCRIPTION OF THE EMBODIMENTS

Referring now to FIG. 1, an optical disk apparatus according to a firstembodiment is described. A laser beam emitted from a laser 108 passesthrough a collimate lens 105 and an objective lens 103 and impinges on apredetermined radial position of a recording medium 101. Reflected lightof the laser beam enters a focusing lens 106 through a beam splitter 104to be focused by the focusing lens 106 and is converted into anelectrical signal (hereinafter referred to as “signal”) by means of aphotoelectric conversion element 107. The signal thus obtained issupplied through an I/V conversion circuit 109 and a signal processingcircuit 110 to a demodulation circuit 111 to be decoded and is sent to ahigher-rank host computer 115 through a microcomputer 114.

In discrimination of the optical disk, a signal necessary todiscriminate the optical disk (hereinafter referred to as “optical diskdiscrimination signal”) is supplied from the signal processing circuit110 to an optical disk discrimination circuit 112. The optical diskdiscrimination signal is different depending on structure of the opticaldisk and contains, for example, a focus error signal and a trackingerror signal.

The discrimination result of the optical disk outputted from the opticaldisk discrimination circuit 112 is supplied through a data bus 116 tothe microcomputer 114. The microcomputer 114 controls the signalprocessing circuit 110, the demodulation circuit 111, a laser driver113, a spindle motor 102 and the like on the basis of the discriminationresult of the optical disk so that control for the discriminated opticaldisk is optimum.

Referring now to FIG. 2, the detailed configuration of the laser driver113 in the optical disk apparatus of FIG. 1 is described. Numeral 201denotes a variable current source controlled by the microcomputer 114through the data bus 116. Numeral 202 denotes a first high-frequencycurrent generation circuit (hereinafter referred to as “OSC1”) and anamplitude and a frequency of the high-frequency current or signalproduced by the OSC1 are described as “HFamp1” and “HFfreq1”,respectively. Numeral 203 denotes a second high-frequency currentgeneration circuit (hereinafter referred to as “OSC2”) and an amplitudeand a frequency of the high-frequency current or signal produced by theOSC2 are described as “HFamp2” and “HFfreq2”, respectively. Numeral 204denotes a signal selection circuit for selecting one of the outputs ofthe OSC1 and the OSC2 to supply it to an adder 205 and the signalselection circuit 204 is controlled by the microcomputer 114 through thedata bus 116. Numeral 205 denotes the adder for adding the outputs ofthe variable current source 201 and the signal selection circuit 204 andthe output of the adder 205 constitutes a laser driving current output117 outputted by the laser driver 113.

Referring now to FIG. 3, the relation of numerical aperture NA of lensand spot diameter R and spot area S formed on the optical disk isdescribed. The spot diameter R is inversely proportional to thenumerical aperture NA as described in the expression 1 and this relationis shown by solid line 301. As described in the expression 2, the spotarea S is inversely proportional to the square of the numerical apertureNA of lens and this relation is shown by solid line 302.

FIG. 6 shows the relation of the laser beam waveform and the laser beampower and the driving current value upon reproduction of the opticaldisk 2. During the period 601 that the high-frequency signal is notsuperposed, the laser power is P2 and the DC laser driving current isC2.

During the period 602 that the high-frequency signal is superposed, thesignal selection circuit 204 shown in FIG. 2 selects the output of theOSC2 to be outputted. In order to make the average laser power duringthe period 602 equal to the laser power P2 during the period 601, C2′satisfying the following relation is required to be set in the variablecurrent source 201 shown in FIG. 2.

Cpk2=C2′+HFamp2  (expression 5)

C2=C2′+(HFamp2/HFfreq2)  (expression 6)

FIG. 7 shows the relation of the laser beam waveform and the laser beampower and the driving current value upon reproduction of the opticaldisk 1. During the period 701 that the high-frequency signal is notsuperposed, the laser power is P1 and the DC laser driving current isC1. The relation of the laser power P1 and the DC laser driving currentC1 of FIG. 7 and the laser power P2 and the DC laser driving current C2of FIG. 6 is given by:

P1<P2  (expression 7)

C1<C2  (expression 8)

Since the laser having the same wavelength and an optical pickup havingthe same optical path length are used for reproduction of the opticaldisks 1 and 2, it is supposed that the signal selection circuit 204shown in FIG. 2 selects the output of the OSC2 to be outputted evenduring the period 702 that the high-frequency signal is superposed. Inorder to make the average laser power during the period 702 equal to thelaser power P1 during the period 701, C1 x satisfying the followingrelation is required to be set in the variable current source 201 shownin FIG. 2.

Cpk2′=C1x+HFamp2  (expression 9)

C1=C1x+(HFamp2/HFfreq2)  (expression 10)

As described in FIG. 4, the optical spot area for the optical disk 1 isabout 0.6 times the optical spot area for the optical disk 2 andaccordingly if the optical disk is irradiated with the same laser power,the laser power per unit area of the optical disk 1 is 1/0.6≈1.7 timesas compared with the optical disk 2. Since the reproduction laser powerC1 (0.3 mW) is set to about 0.6 times of the reproduction laser power C2(0.5 mW), the laser powers per unit area for the average reproductionlaser powers C1 and C2 are substantially equal.

However, when the influence on the increased laser power per unit areaby the HFamp2 superposed during the period 702 is compared with theinfluence on the increased laser power per unit area by the HFamp2superposed during the period 602, the former is larger than the latterand accordingly the laser power per unit area at the high-frequency peakpart during the period 702 is sometimes larger than that during theperiod 602. In other words, there is a possibility that the recordinglayer of the optical disk 1 is irradiated with excessive laser powerdepending on the set value of the HFamp2 and there is a possibility thatthe recorded data is destroyed or the recording layer of the opticaldisk is deteriorated at part 705 shown in FIG. 7.

In order to avoid such problem, the signal selection circuit 204 shownin FIG. 2 selects the output of the OSC1 to be outputted so that therelation of the laser beam waveform, the laser beam power and thedriving current value in the period 703 is satisfied upon reproductionof the optical disk 1. At this time, “HFamp1” and “HFfreq1” outputted bythe OSC1 are set as follows:

HFamp1=HFamp2×(P1/P2)  (expression 11)

HFfreq1=HFfreq2  (expression 12)

When the average beam power P1 and the average driving current C1 arerealized so as to satisfy the above condition, the laser beam waveformby the high-frequency superposition is similar to that in the period 602of FIG. 6 as shown in the period 703 of FIG. 7 and the laser powers perunit area for the high-frequency superposed peak power Ppk1 and thehigh-frequency superposed peak power Ppk2 of FIG. 6 are substantiallyequal to each other.

FIG. 5 shows the relation of the laser driving current and the laserbeam power by the laser driving currents (hereinafter referred to as“I/L relation”). Solid line 501 shows the relation of the average laserpower and the average driving current in the period that thehigh-frequency signal is not superposed on the laser driving current asshown in the period 603 of FIG. 6 and the period 703 of FIG. 7. Further,solid line 502 shows the relation of the average laser power and theaverage laser driving current in the period that the high-frequencysignal is superposed on the laser driving current as shown in the period602 of FIG. 6, the periods 702 and 703 of FIG. 7, for example. Solidline 503 shows the relation of the high-frequency superposed peak powervalue and the laser driving current at that time.

When the laser beam waveforms upon the high-frequency superposition aresimilar to each other as shown in the periods 602 and 703, the powervalues such as the peak laser power average value of the superposedhigh-frequency signal are put on the same straight line in the areawhere the laser power is changed linearly to the driving current.Accordingly, the relation of I/L at each level of Ppk1 and Ppk2 ispreviously calculated and the laser power and the laser driving currentvalue at each level may be set in accordance with the laser spotdiameter on the recording surface of the optical disk to be reproduced.

FIG. 8 is a flow chart showing processing of the embodiment. When theoptical disk is mounted in the optical disk apparatus (step 801), anoptical pickup head is moved to a predetermined reproduction position inorder to prevent data recorded on the optical disk from being destroyed(step 802) and setting of reproduction for the optical disk 2 is made asdescribed by expressions 13, 14 and 15 (step 803). Then, it is judgedwhether the optical disk is the optical disk 2 or not (step 804). Atthis time, the signal selection circuit 204 shown in FIG. 2 selects theoutput of the OSC2 to be outputted.

Numerical Aperture=NA2  (expression 13)

Reproduction Power=P2  (expression 14)

High-Frequency Superposed Peak Power=Ppk2  (expression 15)

The reason why the kind of the optical disk is discriminated in thesetting of reproduction for the optical disk 2 is that the optical spotdiameter for the optical disk 2 is larger than that for the optical disk1 and accordingly the laser power per unit area on the optical disk isreduced even if the high-frequency superposed peak power is the same, sothat destruction of the recorded data can be avoided.

In judgment step 804, when the optical disk is judged to be the opticaldisk 2 (step 805), the reproduction processing of data is started inaccordance with the setting defined by the expressions 5 and 6 (step807).

In judgment step 804, when the optical disk is judged not to be theoptical disk 2, that is, when it is judged to be the optical disk 1(step 806), the reproduction condition of the optical disk 1 is set inaccordance with the expressions 9 and 10 and the following expressionsand the reproduction processing is started (step 807).

Numerical Aperture=NA1  (expression 16)

Reproduction Power=P1  (expression 17)

High-Frequency Superposed Peak Power=Ppk1  (expression 18)

In the embodiment, the optical disk 1 conforming to the BD standards andthe optical disk 2 conforming to the HD-DVD standards are illustrated asthe optical disks having the same laser wavelength to be reproduced anddifferent reproduction laser spot areas, although it is needless to saythat the present invention can be applied even to the optical diskshaving the same relation.

Further, in the embodiment, the amplitude “HFamp1” of the high-frequencysuperposed signal of the optical disk 1 is calculated by the expression11, although the amplitude HFamp1 satisfying the following conditionsmay be set:

1. when the optical disk is rewritable, the high-frequency superposedpeak power Ppk1 is smaller than erasing power; and2. when the optical disk is of the read only type or of once recordabletype, the high-frequency superposed peak power Ppk1 is power by whichthe data recording layer is not destroyed.

In FIG. 2, the OSC1 (202), the OSC2 (203) and the signal selectioncircuit 204 are disposed within the laser driver 113, although any orall of them may be disposed outside of the laser driver 113. Accordingto such configuration, the laser driver can be made small and the heatgeneration amount can be reduced.

Further, the processing in step 802 of FIG. 8 may be omitted.Consequently, the judgment or discrimination time of the disk can beshortened.

In step 803 of FIG. 8, the optical disk is judged or discriminated afterthe setting of the high-frequency superposition, although the opticaldisk may be discriminated without superposing the high-frequency signal.In this case, the reproduction power P2 and the high-frequencysuperposed peak power Ppk2 may be set in step 805.

Referring now to FIG. 13, a second embodiment is described. In thisembodiment, there is provided a high-frequency current generationcircuit 1301 which can set a high-frequency current amplitude and highfrequency from the outside of the laser driver, so that the amplitudeand the frequency of the high-frequency signal to be superposed can bechanged in accordance with the discrimination result of the optical diskinstead of the plurality of high-frequency current generation circuits(OSC1, OSC2) provided in the laser driver as shown in FIG. 2 of thefirst embodiment. The configuration of the optical disk apparatus of thesecond embodiment is the same as that of the first embodiment as shownin FIG. 1 and accordingly description thereof is omitted.

FIG. 13 is a block diagram schematically illustrating an internalconfiguration of the laser driver 113 of the embodiment. Description ofthe elements 201 and 205 common to those in FIG. 2 of the firstembodiment is omitted. The high-frequency current generation circuit1301 includes a current amplitude control circuit 1302 for controlling acurrent amplitude of the high-frequency signal and a frequency controlcircuit 1303 for controlling the frequency of the high-frequency signaland both of them are controlled by the microcomputer 114 through thedata bus 116. A control value of variable current source 201, thecurrent amplitude control circuit 1302 is set so that the average of thereproduction laser power is equal to the optimum reproduction laserpower in accordance with the kind of the optical disk discriminated bythe optical disk discrimination circuit 112.

For example, when the optical disk is judged to be the optical disk 1,the setting for the current amplitude control part 1302 is set to HFamp1and when the optical disk is judged to be the optical disk 2, thesetting for the current amplitude control part 1302 is set to HFamp2. Atthis time, when the setting for the frequency control part 1303 is setto be HFfreq2=HFfreq1 in either case, the same effects as in the firstembodiment can be attained.

A third embodiment of the present invention is now described.

FIG. 9 is a schematic diagram illustrating an optical disk apparatusaccording to a third embodiment of the present invention. The likeelements to those shown in FIG. 1 are designated by like numerals anddescription thereof is omitted. Numeral 901 denotes a monitor diode fordetecting laser emission power in order to perform the automatic powercontrol (APC) and the signal band of the monitor diode is sufficientlylow as compared with the high-frequency signal superposed on thereproduction laser beam. The signal 902 detected by the monitor diode901 is supplied to a laser power control circuit 903. The detailedconfiguration of the laser power control circuit 903 and a laser driver905 is shown in FIG. 10. The like elements to those shown in FIG. 13 aredesignated by like numerals and description thereof is omitted.

In the laser power control circuit 903, a reproduction laser powertarget value corresponding to the optical disk is set to a reproductionpower target value generation circuit 1001 by the microcomputer 114 inaccordance with the discrimination result of the optical disk by theoptical disk discrimination circuit 112. A difference between this setvalue and an output 902 of the monitor diode is calculated by asubtractor 1002 to be outputted as a difference value 904. Thereproduction laser driving current is generated on the basis of thedifference value 904 through an amplifier 1003. Further, in ahigh-frequency current generation circuit 1004, a variable gainamplifier 1005 controlled by the difference value 904 controls theamplitude set value with respect to the amplitude value set in thehigh-frequency amplitude control circuit 1302 in accordance with theoutput of the optical disk discrimination circuit 112. The coefficientratio of the variable gain amplifier to the difference value 904 may bethe ratio of values shown by the solid lines 502 and 503 in FIG. 5 ofthe first embodiment, for example. Consequently, satisfactoryhigh-frequency superposition corresponding to the optical diskreproduction power can be realized and mis-erasure of data uponreproduction can be prevented while correcting change in the relation ofI/L due to change in temperature within the optical disk apparatus andat the periphery of laser and deterioration due to aging.

FIG. 11 is a flow chart showing processing of the embodiment. When theoptical disk is mounted (step 1101), the setting for the target value ofthe reproduction power target value generation circuit 1001 is set to P1or P2 of FIG. 5 and the setting for the amplitude and the frequency ofthe high-frequency current generation circuit 1004 are set in common tothe first and second optical disks 1, 2. In the embodiment, the settingbased on the blue laser is made and a switch 1006 shown in FIG. 10 isturned off, so that the output of the high-frequency current generationcircuit 1004 is cut off. In this state, laser noise is superposed on thereproduction signal and accordingly the signal quality (S/N) isdeteriorated, although the signal band of the focus error signal and thetracking error signal used mainly in discrimination of the optical diskis as sufficiently low as several kHz, so that the signal quality can berelatively easily improved by means of a low-pass-filter (LPF) or thelike. By using the signal having the quality thus improved, the opticaldisk can be discriminated with performance of the same degree as that ofthe discrimination of the optical disk using the reproduction waveformon which the high-frequency signal is superposed. The high-frequencysignal is not superposed as described above, so that mis-erasure of therecorded data on the optical disk and deterioration of the recordinglayer of the optical disk at the time that the peak power of the laserbeam is excessive upon the high-frequency superposition can be avoided.In this state, discrimination of the optical disk is carried out (step1102) and when the optical disk is judged to be the optical disk 2 (Yesof step 1103), the target value of the reproduction power target valuegeneration circuit 1001 of FIG. 10 is set to the reproduction power P2of the optical disk 2 (step 1105). When it is judged not to be theoptical disk 2, that is, when it is judged to be the optical disk 1 (Noof step 1103), the target value of the reproduction power target valuegeneration circuit 1001 of FIG. 10 is set to the reproduction power P1of the optical disk 1 (step 1104). Then, the output of thehigh-frequency current generation circuit 1004 is made effective by theswitch 1006 (step 1106) and the reproduction processing is started (step1107).

FIG. 12 is a flow chart showing processing of a fourth embodiment of thepresent invention. The circuit configuration of the embodiment is thesame as that of the third embodiment and description thereof is omitted.In the embodiment, there is considered the case where the numericalaperture NA in reproduction is different in each layer of a multi-layeroptical disk in which the blue laser is used to reproduce information.When the optical disk is mounted (step 1201) or when the reproductionlayer of the optical layer is changed (step 1202), the following settingis made (step 1203) as preparation for reproduction of a layer to bereproduced (hereinafter referred to as “target layer”):

-   -   NA of the target layer=NAt    -   the reproduction power Pt of the target layer is set to the        target value of the reproduction power target value generation        circuit 1001 of FIG. 10.

At this time, the switch 1006 of FIG. 10 is turned off, so that theoutput of the high-frequency current generation circuit is cut off.Then, focusing on the target layer is made (step 1204). When thefocusing on the target layer can be confirmed, the switch 1006 is turnedon to make effective the output of the high-frequency current generationcircuit (step 1205). Then, start tracking control (step 1206) and thereproduction processing is started (step 1207). Consequently,destruction of the data recorded on the layer other than the targetlayer or deterioration of the recording layer caused by thehigh-frequency superposed peak power upon the focusing can be prevented.

In the embodiments, the high-frequency signal is superposed on the laserdriving signal upon reproduction, although it is needless to say thatthe present invention described in the embodiments can be applied evenwhen reproduction is made for generation of a servo signal in recordingor when the laser beam of the reproduction power level is emitted uponformation of space in recording.

Further, in the embodiments, the characteristics of the high-frequencysignal superposed on the laser driving signal upon reproduction arechanged, although the output of the high-frequency signal may be stoppedif circumstances require, that is, the high-frequency signal having anamplitude reduced to zero may be used and the high-frequency signal isnot limited to the waveform as shown in FIGS. 6 and 7.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1-19. (canceled)
 20. An optical disc apparatus which reproducesinformation recorded on a first recoding layer disposed at a firstdistance from a surface of an optical disc having a plurality ofrecoding layers and information recorded on a second recording layerdisposed at a second distance from the surface of the optical disc,comprising: a laser which emits a laser beam having a predeterminedwavelength; an objective lens which focuses the laser beam having thepredetermined wavelength emitted from the laser on the first recodinglayer and the second recording layer of the optical disc; a laser driverwhich supplies a laser driving current on which a high-frequency currenthaving a first amplitude is superposed to the laser when reproducinginformation recorded on the first recoding layer and supplies a laserdriving current on which a high-frequency current having a secondamplitude which is different from the first amplitude is superposed tothe laser when reproducing information recorded on the second recodinglayer; and a laser power control circuit which controls a laser power ofthe laser such that the laser having a first laser power is emitted tothe first recording layer when supplying a laser driving current onwhich a high-frequency current having the first amplitude is superposedto the laser and controls a laser power of the laser such that the laserhaving a second laser power which is different from the first laserpower is emitted to the second recording layer when supplying a laserdriving current on which a high-frequency current having the secondamplitude is superposed to the laser.
 21. An optical disk apparatusaccording to claim 20, wherein the first amplitude is lower than thesecond amplitude; the first laser power is lower than the second laserpower; and the laser driver supplies the laser driving current on whicha high-frequency current having the second amplitude is superposed tothe laser and the laser power control circuit controls the laser powerof the laser such that the laser having the second laser power isemitted to the second recording layer after reproducing informationrecorded on the first recoding layer.
 22. An optical disk apparatusaccording to claim 21, wherein an allowed laser power of the firstrecording layer is higher than an allowed laser power of the secondrecording layer.
 23. An optical disc reproduction method of reproducinginformation recorded on a first recording layer disposed at a firstdistance from a surface of an optical disc having a plurality ofrecoding layers and information recorded on a second recording layerdisposed at a second distance from the surface of the optical disc,comprising: emitting a laser beam having a predetermined wavelength;focusing the laser beam having the predetermined wavelength emitted froma laser on the first recoding layer and the second recording layer ofthe optical disc; supplying a laser driving current on which ahigh-frequency current having a first amplitude is superposed to thelaser and controlling a laser power of the laser such that the laserhaving a first laser power is emitted to the first recording layer whenreproducing information recorded on the first recording layer; supplyinga laser driving current on which a high-frequency current having asecond amplitude is superposed to the laser and controlling a laserpower of the laser such that the laser having a second laser power isemitted to the second recording layer when reproducing informationrecorded on the second recording layer.
 24. An optical disc reproductionmethod according to claim 23, wherein the first amplitude is lower thanthe second amplitude; the first laser power is lower than the secondlaser power; and supplying the laser driving current on which ahigh-frequency current having the second amplitude is superposed to thelaser and controlling the laser power of the laser such that the laserhaving the second laser power is emitted to the second recording layerafter reproducing information recorded on the first recoding layer. 25.An optical disc reproduction method according to claim 24, wherein anallowed laser power of the first recording layer is higher than anallowed laser power of the second recording layer.
 26. An optical discapparatus which reproduces information recorded on a first recodinglayer disposed at a first distance from a surface of an optical dischaving a plurality of recoding layers and information recorded on asecond recording layer disposed at a second distance from the surface ofthe optical disc, comprising: a laser which emits a laser beam having apredetermined wavelength; an objective lens which focuses the laser beamhaving the predetermined wavelength emitted from the laser on the firstrecoding layer and the second recording layer of the optical disc; ahigh-frequency current generation circuit which generates ahigh-frequency current having a first amplitude and a second amplitude;a laser power control circuit which controls a laser power of the laser;wherein the laser having a first laser power is emitted to the firstrecording layer when reproducing information recorded on the firstrecoding layer and the laser having a second power is emitted to thesecond recording layer when reproducing information recorded on thesecond recoding layer; and wherein a high-frequency current having thefirst amplitude is superposed to the laser having the first laser power,and a high-frequency current having the second amplitude is superposedto the laser having the second laser power.
 27. An optical diskapparatus according to claim 26, wherein the first amplitude is lowerthan the second amplitude; the first laser power is lower than thesecond laser power; and the laser having a second power is emitted tothe second recording layer after reproducing information recorded on thefirst recoding layer.
 28. An optical disk apparatus according to claim27, wherein an allowed laser power of the first recording layer ishigher than an allowed laser power of the second recording layer.
 29. Anoptical disc reproduction method of reproducing information recorded ona first recording layer disposed at a first distance from a surface ofan optical disc having a plurality of recoding layers and informationrecorded on a second recording layer disposed at a second distance fromthe surface of the optical disc, comprising: emitting a laser beamhaving a predetermined wavelength; focusing the laser beam having thepredetermined wavelength emitted from a laser on the first recodinglayer and the second recording layer of the optical disc; generating ahigh-frequency current having a first amplitude and a second amplitude;controlling a laser power of the laser; wherein the laser having a firstlaser power is emitted to the first recording layer when reproducinginformation recorded on the first recoding layer and the laser having asecond power is emitted to the second recording layer when reproducinginformation recorded on the second recoding layer; and wherein ahigh-frequency current having the first amplitude is superposed to thelaser having the first laser power, and a high-frequency current havingthe second amplitude is superposed to the laser having the second laserpower.
 30. An optical disk apparatus according to claim 29, wherein thefirst amplitude is lower than the second amplitude; the first laserpower is lower than the second laser power; and the laser having asecond power is emitted to the second recording layer after reproducinginformation recorded on the first recoding layer.
 31. An optical diskapparatus according to claim 30, wherein an allowed laser power of thefirst recording layer is higher than an allowed laser power of thesecond recording layer.